The present invention relates to a novel multi-ring die for extruding tubes composed of a plurality of layers, and more particularly to a multi-ring die having a die portion which is composed of a core and a plurality of die rings lying one over another and a column-like portion formed with resin flow passages for feeding resins into the die portion.
Previous ordinary extrusion dies for the manufacture of tubes composed of a plurality of layers are provided with resin supplying passages which comprise an inmost passage for feeding a resin of inner layer disposed on the center line of the die, a second circular passage surrounding the inmost passage concentrically therewith for feeding a resin of a second layer and other circular passages lying one around another for feeding resins of respective layers. However, the die of this type has some deficiences.
Firstly, although the resin for the inmost layer can be supplyed through the center of the die, the layers other than the inmost layer are required to be supplying in directions radially to the center line of the die through the nozzle provided at a side thereof, striding over the passages internaly disposed. In the die of this type, the resin fed laterally diverges by means of a manifold and flows, encircling inside the manifold and thereafter turning its direction to the direction which is parallel to the axis of the die, into the inlet of the die passage through the narrow lands. In the case, the land which resist to the resin flow should be so designed that the land near the inlet of resin is made longer and on the contrary the land near far end of the manifold is shortened to compensate pressure depression due to resin flow through the manifold, so as to equalize the amount of resin flow per unit area around the die passage.
The structure of the manifold and the land is designed so that it is in conformity with a viscosity index of a plasticized resin, and on the other hand viscosity index thereof is variable depending upon the resin temperature and the shearing velocity during the process of formation. Therefore, in the ordinary die stated above having an inlet provided in the sides thereof, the unevenness of the resin flow per unit area around the circular die passage occurs when one of the extrusion conditions such as species of resin, resin temperature, the amount of resin flow or the like is altered, even though a construction concerning a manifold and the like is suitably determined where uniform and even resin flow around the circular die passage is obtainable under certain extrusion conditions.
Secondly, in the previous die assembly having nozzles equipped at the side thereof, the passages of resin should be so designed that the passages other than that for the inmost layer stride over the passages internally disposed and that the passages change the directions thereof gradually from radial directions to the direction which are parallel to the axis of the die. In the die of this type, since the diameter of the outer die passage becomes large with increase of the numbers of the layers, the portion of the die where the outer passage is curved from radial direction to the axial direction is proportionally increased both in radial and axial, i.e. parallel dimensions. As is apparent from the above descriptions, the dies of the ordinary ones are not only expensive to make and large in volume but also inconvenient to handle.